The assessment of fluid flow in the human body is important for medical diagnosis. For example, blood velocity and the volumetric flow (i.e., the volume of blood flowing through a blood vessel, e.g., in liter per second) routinely assist clinical decisions.
Various ultrasound techniques can be used to measure the motion of scattering materials such as blood, body fluids and tissue. Ultrasound contrast agents can also be used to enhance signals from fluids with insufficient scatter properties. For example, blood velocity can be measured in a small volume using the Doppler principle. In echographic B-scanning, multiple estimates of blood velocity in the plane of the scan can be combined with the gray-scale echo image by colouring.
Miniaturised ultrasound transducers can be placed inside the lumen of a vessel or other body cavities to obtain a cross-sectional echo image. The same ultrasound echo signals can be used to measure the velocity of the flowing blood or other fluids.
The purpose of the invention is to provide a method for measuring volumetric fluid flow and its velocity profile in a lumen or other body cavity. According to the invention an ultrasonic method is provided to measure volumetric flow through a lumen by accomplishing simultaneously and in situ (in place) the steps of
a) measuring the local velocity of the scattering medium perpendicular to the ultrasound scan plane and
b) integrating such velocity measurements over the area of the lumen.
An approximation to the above method in accordance with the invention comprises the steps of
a) calculating the average value of the velocity of the scattering medium perpendicular to the ultrasound scan plane,
b) calculation of the area of flow, and
c) multiplying the average velocity by the area to obtain volume flow.
Furthermore, in order to reduce the number of calculations, the average velocity can be approximated by measurement of the velocity in a sub-region of the area of flow smaller than the total area of flow, and volume flow can be computed as before.
In the method according to the invention the scattering fluid of interest may be blood. For purposes of explaining the invention, the invention shall be discussed in relation to blood. Other fluids may be measured in a similar manner. Blood is composed by red blood cells (RBCs), white blood cells and platelets suspended in a liquid called plasma. Because the size and density of RBCs is large compared to that of white cells and platelets, backscatter of blood is attributed to the red blood cells. The measurement of blood velocity comprises the steps of:
a) obtain (transmit pulse and receive echo) two or more subsequent echo signals from a single (or a slightly changed) position of the ultrasound transducer at controlled interval(s) of time xcex94t,
b) measure one or more displacements of the blood relative to the beam, xcex94d, and
c) compute velocity from the ratios of displacements over the time interval, v=xcex94d/xcex94t.
One embodiment of the invention relates to the measurement of volumetric flow and velocity imaging from within the lumen of a blood vessels using intravascular ultrasound. It has to be mentioned here that conventional ultrasound has already been proposed to measure blood velocity on the plane of the scan. The scan is usually oriented along the blood vessel. If the scan plane is oriented perpendicular to the blood vessel, volumetric flow could also be computed as describe hereinafter.